This invention relates generally to substrates for use in Ball Grid Array integrated packages. More particularly, it relates to a technique which more efficiently utilizes the area of Ball Grid Array substrates.
As technology advances, the functionality, complexity and speed of integrated circuit (IC) chips are steadily increasing. These increasingly complex, high speed IC chips often require correspondingly increasing numbers of electrical interconnections in which to communicate with other components. Consequently, high density interconnect package assemblies and the trend toward miniaturization have been principal objectives of the semiconductor industry. One such package developed to meet these objectives is the Ball Grid Array (BGA) integrated assembly.
FIG. 1 shows a cross section of a high interconnect density BGA package, generally referred to by reference number 10. The package includes an integrated circuit die 12 affixed to and supported by a dielectric substrate 14. Die 12 includes a plurality of bond pads (not shown) which are connected to the active circuitry within the die. Substrate 14 includes a plurality of conductive traces 16 that are formed from a thin solder mask layer disposed on the top surface of the substrate 14. The conductive traces 16 are subsequently formed for connection with bonding wires 18 that are connected to the bond pads of the die 12. On the bottom surface of substrate 14 are a multiplicity of solder balls 20 attached thereto in a dense grid pattern. The solder balls 20 are arranged to be received on a circuit board having matching electrical contacts. This dense grid arrangement serves to provide the package with electrical communication to the outside world and is the basis for the high interconnect density of the BGA package. The conductive traces 16 are connected to the solder balls 20 by way of vias (not shown) routed through or around the substrate 14. A plastic material is molded over the device, which in cooperation with the substrate 14, encapsulates the device to protect it from the outside environment.
With the basic BGA structure given above, it is generally the case that the substrate constitutes a major part of the cost (about 85% or so) of the package. This is especially true of substrates constructed of Bismaleimid Triazine (BT), which is a relatively expensive material. The advantage of BT is that it possesses a relatively high glass transition temperature (T.sub.g) of about 180.degree. C. as opposed to FR4 which is widely used for printed circuit boards which has a glass transition temperature of about 150.degree. C., for example. A high glass transition temperature is required during manufacturing where the molding process temperatures can reach 170.degree.-180.degree. C.
FIG. 2 illustrates a typical substrate used in BGA packages generally designated by reference numeral 24. Substrates are generally supplied in strip form with typically anywhere from 3-7 units per strip. A strip configuration is used because it is more conducive to processing by automation where the equipment can process multiple units efficiently as opposed to individual processing. The particular substrate shown in FIG. 2 is supplied by Shinko Electric of Japan designated as part number 313 BGA. Holes 26 are punched near the edges of strip 24 so that the processing equipment can grasp and align the part during manufacturing. Stress relief slots 28 are punched periodically between units 30 which allow for expansion when subjected to the high processing temperatures during the molding operation. It can be seen that two slots are punched in between units thereby leaving material area 32 unused. After the packages are formed, the final step is to separate the units 30 from the strip by a punching operation that punches out the part all the way around which is referred to as singulation or separation. Since the BT substrate material is very tough, the singulation step can result in substrate and solder mask cracking and depart undue wear and stress on the punching apparatus and possibly shorten tool life prematurely. As the substrate thickness increases, the problems described above only get worse.
FIG. 2a illustrates a substrate used in BGA packages manufactured by Anam of Industrial Co. of Korea. In contrast to the Shinko design, single stress relief slots 28 are located between and above and below units 30. A similar manufacturing process is performed as with the Shinko substrate in FIG. 2 where the final step is to punch out the individual packages. Index holes 27 are used for the purpose of positioning the punch to the correct location around the unit prior to punching out the package. Since the packages are punched out all the way around, the same shortcomings exist as in the previous substrate 24.
With the relative high cost of substrate material and undue wear imposed during the punching operation, what is needed is a more efficient use of substrate area so that less material is wasted during manufacturing. What is further needed is a simplified punching operation that reduces the stresses inflicted on the substrate and punching equipment to reduce the possibility of substrate damage. As will be described hereinafter, the present invention, in accordance with the above objectives, may yield significant savings in the cost of manufacturing such a package.